A communication system located in an electromagnetism noisy environment is isolated electromagnetically from the environment exterior to the communication system by using shielded cables, and electromagnetic interference emissions from cables are suppressed in the communication system. A shielded cable is comprised of an inner conducting wire, an outer conductor for shield (an outer conductor sheath), and a holding resin. The inner conducting wire is connected to a signal system and the outer conductor sheath is connected to a ground in such a way that a shielding structure in which the internal signal system is enclosed by the ground is formed.
However, in many cases, equipment for use in a heavy current system which handles large electric power is constructed in such a way that a ground which is a return circuit for a large current and a ground for conductor wires in a weak electric current system are separated from each other from the viewpoint of electrical safety, and the grounds are provided as systems which are separated from each other. For example, a metal housing or the like which is aground for a return circuit in a heavy current system is defined as a frame ground (FG), and a ground for a signal system connected to an outer conductor of a shielded cable is defined as a signal ground (SG). Because these FG system and SG system are separated from each other, when an electromagnetism noise is piggybacked onto SG, there is a possibility that the noise propagates through the outer conductor of the shielded cable and affects various pieces of equipment. Therefore, although it is necessary to connect the SG system to the FG system to bypass the electromagnetism noise, the SG system cannot be connected directly to the FG system from problems about electrical safety.
To solve this problem, a structure in which no conduction from an SG system to an FG system is provided, but only an electromagnetism noise occurring in the SG system is bypassed is disclosed. For example, according to patent reference 1, a dielectric substance is disposed around a BNC type plug (a connector), and metallic mounting hardware is disposed around the dielectric substance. Further, according to, for example, patent reference 2, a plate capacitor is disposed between a frame and the locknuts of a connector. In addition, according to, for example, patent reference 3, a structure in which an O ring, which is conventionally inserted between a locknut and a chassis, is replaced by a filtering device (a capacitor), such as a capacitor, is disclosed. The outer conductor of a connector is electrically connected to a frame via the filtering device. In addition, according to, for example, patent reference 4, the outer conductor of a connector is electrically connected to a frame via a capacitor and a resistor. In addition, according to, for example, patent reference 5, a connector for a coaxial cable having multiple outer conductors is constructed in such a way that an inside outer conductor and an outside outer conductor are terminated respectively.